Sorry, but I don't see the Attachements tab.I only see Post icon, below Subject,icon for Bold/Italic/..., a frame where I'm typing my reply, 3 buttons Save draft/Preview/Submit, then some options, but no Attachments tab.Can you make a screenshot showing me where I can see it ?

If the Attachments tab is not available to standard users, then I can only suggest that you compare the settings in IncrediMail and Thunderbird and make sure the settings in IncrediMail exactly match those in Thunderbird.